1. Field of the Invention
The present invention relates to a high pass filter, and particularly relates to a high pass filter including an LC series resonator.
2. Description of the Related Art
A high pass filter is described in Japanese Unexamined Patent Application Publication No. 2008-167157. FIG. 8 is an equivalent circuit diagram of a high pass filter 500 described in Japanese Unexamined Patent Application Publication No. 2008-167157. FIG. 9 is an exploded perspective view of the high pass filter 500 shown in FIG. 8.
As shown in FIG. 8, the high pass filter 500 includes input and output terminals 504 and 506; capacitors C201, C202, and C203; and LC series resonators LC101 and LC102. The capacitors C201, C202, and C203 are connected in series in this order between the input and output terminal 504 and the input and output terminal 506. The LC series resonator LC101 includes an inductor L101 and a capacitor C101. One end of the LC series resonator LC101 is connected between the capacitor C201 and the capacitor C202, whereas the other end of the LC series resonator LC101 is connected to ground. The LC series resonator LC102 includes an inductor L102 and a capacitor C102. One end of the LC series resonator LC102 is connected between the capacitor C202 and the capacitor C203, whereas the other end of the LC series resonator LC102 is connected to ground.
As shown in FIG. 9, the high pass filter 500 includes a multilayer body 502. The multilayer body 502 has a structure formed by stacking a plurality of insulator layers. The inductor L101 is positioned in the multilayer body 502, and is substantially in the shape of a helix that extends downward while being wound in a counterclockwise direction when viewed from above. The inductor L102 is positioned in the multilayer body 502, and is substantially in the shape of a helix that extends downward while being wound in a clockwise direction when viewed from above. The capacitor C101 is positioned near a lower surface of the multilayer body 502 and connected to the inductor L101. The capacitor C102 is positioned near the lower surface of the multilayer body 502 and connected to the inductor L102.
The high pass filter 500 has approximately two attenuation poles in the bandpass characteristic, as shown in the graph of FIG. 5 in Japanese Unexamined Patent Application Publication No. 2008-167157.
The approximately two attenuation poles of the high pass filter 500 may need to be brought close to each other. In such a case, for example, the shape of one of the inductor L101 and the inductor L102 may be changed to adjust the inductance value.
However, changing the shape of one of the inductor L101 and the inductor L102 breaks the symmetric structure of the high pass filter 500. This causes a difference between the bandpass characteristic obtained when the input and output terminal 504 is used as an input terminal and the input and output terminal 506 is used as an output terminal, and the bandpass characteristic obtained when the input and output terminal 506 is used as an input terminal and the input and output terminal 504 is used as an output terminal. As a result, the symmetry of input and output is changed or destroyed. An inductor or capacitor for impedance matching may be added to the high pass filter 500. However, this leads to an increased device size.